The field of the invention pertains to internal combustion engines and, in particular, to fiber optic sensors for instantaneous cylinder pressure in operating engines.
An automatic cylinder pressure sensor mounted under the hood of a vehicle is exposed to widely fluctuating temperatures both in the transducer tip area as well as in the sensor signal conditioner location. Current signal conditioner specifications require operation at xe2x88x9240xc2x0 C. to up to 120-150xc2x0 C., depending on conditioner mounting location, while the transducer tip temperature is specified in the range of xe2x88x9240xc2x0 C. to 300-350xc2x0 C. During normal continuous engine operation the transducer temperature is typically in the 150-250xc2x0 C. range. During a cold engine start the transducer temperature change from xe2x88x9240xc2x0 C. to its operating temperature occurs in a matter of several seconds.
During a vehicle lifetime, under hood temperatures over 125xc2x0 C. occur relatively rarely and in special situations such as prolonged driving at maximum load (e.g., up hill towing) or in a hot climate (e.g., Arizona). In addition, the maximum temperature varies significantly depending on under hood location. Mounting near engine exhaust ports may expose a sensor signal conditioner to radiant heat in addition to ambient temperatures, whereas, in contrast locating a sensor signal conditioner near an air intake may reduce peak temperatures by as much as 20-30xc2x0 C.
Unless protected and compensated, temperatures over 125xc2x0 C. may result in the failure of an LED light source in the sensor signal conditioner for the fiber optic sensor. While a maximum LED storage temperature is typically 150xc2x0 C., the maximum operating temperature must be lower so that the LED p-n junction temperature does not exceed 150xc2x0 C. Typically, the LED current heats the junction leading to a differential between ambient and junction temperature of 10-20xc2x0 C. When an LED junction reaches 150xc2x0 C. rapidly growing defects result in output power reduction and potential LED failure. While in some applications an LED can be thermoelectrically cooled, in high volume automotive use the cost constraints prohibit use of any such cooling devices.
This invention comprises several associated techniques intended to protect a diaphragm type fiber optic cylinder pressure sensor from the effects of maximum under hood temperatures and to minimize errors associated with rapidly changing under hood and engine temperatures. In particular, the invention:(i) reduces maximum LED temperature by attaching the sensor signal conditioner to a fuel line; (ii) limits LED junction temperature to less than 150xc2x0 C. through xe2x80x9cintelligentxe2x80x9d management of LED current based on under hood temperature; (iii) minimizes errors associated with sensor temperature changes occurring during the transition between cold engine cranking and engine firing; (iv) reduces temperature-related errors due to fluctuations in the coupling efficiency between optical fibers and the LED (or photodiode); and (v) minimizes the thermal shock error associated with rapidly varying cylinder gas temperatures.
The techniques include electronic compensation in response to temperature change, fuel system cooling of the optoelectronic interface, construction of the interface and construction of the sensor tip.